Molecular structure of antibodies against small ligands

Antibodies which bind bioactive ligands can serve as a template for the generation of a second antibody which may react with the physiological receptor. This phenomenon of molecular mimicry by antibodies has been described in a variety of systems. In order to understand the chemical and molecular mechanisms involved in these interactions, monoclonal antibodies directed against two pharmacologically active alkaloids, morphine and nicotine, were carefully studied using experimental and theoretical molecular modeling techniques. The molecular characterization of these antibodies involved binding studies with ligand analogs and determination of the variable region amino acid sequence. A three-dimensional model of the anti-morphine binding site was constructed using computational and graphics display techniques. The antibody response in BALB/c mice to morphine appears relatively restricted, in that all of the antibodies examined in this study contained a $\lambda$ light chain, which is normally found in only 5% of mouse immunoglobulins. This study represents the first use of theoretical and experimental modeling techniques to describe the antigen binding site of a mouse Fv region containing a $\lambda$ light chain. The binding site model indicates that a charged glutamic acid residue and aromatic side chains are key features in ionic and hydrophobic interactions with the ligand morphine. A glutamic acid residue is found in the identical position in the anti-nicotine antibody and may play a role in binding nicotine. ^

Molecular mechanisms of prostacyclin receptor signaling: The intracellular domains in G protein coupling

To better understand the mechanisms of how the human prostacyclin receptor (1P) mediates vasodilation and platelet anti-aggregation through Gs protein coupling, a strategy integrating multiple approaches including high resolution NMR experiments, synthetic peptide, fluorescence spectroscopy, molecular modeling, and recombinant protein was developed and used to characterize the structure/function relationship of important segments and residues of the IP receptor and the α-subunit of the Gs protein (Gαs). The first (iLP1) and third (iLP3) intracellular loops of the IP receptor, as well as the Gαs C-terminal domain, relevant to the Gs-mediated IP receptor signaling, were first identified by observation of the effects of the mini gene-expressed corresponding protein segments in HEK293 cells which co-expressed the receptor and Gαs. Evidence of the IP iLP1 domain interacted with the Gαs C-terminal domain was observed by fluorescence and NMR spectroscopic studies using a constrained synthetic peptide, which mimicked the IP iLP1 domain, and the synthetic peptide, which mimicked Gαs C-terminal domain. The solution structural models and the peptide-peptide interaction of the two synthetic protein segments were determined by high resolution NMR spectroscopy. The important residues in the corresponding domains of the IP receptor and the Gαs predicted by NMR chemical shift mapping were used to guide the identification of their protein-protein interaction in cells. A profile of the residues Arg42 - Ala48 of the IP iLP1 domain and the three residues Glu392 ∼ Leu394 of the Gαs C-terminal domain involved in the IP/Gs protein coupling were confirmed by recombinant proteins. The data revealed an intriguing speculation on the mechanisms of how the signal of the ligand-activated IP receptor is transmitted to the Gs protein in regulating vascular functions and homeostasis, and also provided substantial insights into other prostanoid receptor signaling. ^

Modeling Techniques for the High-Resolution Interpretation of Cryo-Electron Microscopy Reconstructions

Essential biological processes are governed by organized, dynamic interactions between multiple biomolecular systems. Complexes are thus formed to enable the biological function and get dissembled as the process is completed. Examples of such processes include the translation of the messenger RNA into protein by the ribosome, the folding of proteins by chaperonins or the entry of viruses in host cells. Understanding these fundamental processes by characterizing the molecular mechanisms that enable then, would allow the (better) design of therapies and drugs. Such molecular mechanisms may be revealed trough the structural elucidation of the biomolecular assemblies at the core of these processes. Various experimental techniques may be applied to investigate the molecular architecture of biomolecular assemblies. High-resolution techniques, such as X-ray crystallography, may solve the atomic structure of the system, but are typically constrained to biomolecules of reduced flexibility and dimensions. In particular, X-ray crystallography requires the sample to form a three dimensional (3D) crystal lattice which is technically di‑cult, if not impossible, to obtain, especially for large, dynamic systems. Often these techniques solve the structure of the different constituent components within the assembly, but encounter difficulties when investigating the entire system. On the other hand, imaging techniques, such as cryo-electron microscopy (cryo-EM), are able to depict large systems in near-native environment, without requiring the formation of crystals. The structures solved by cryo-EM cover a wide range of resolutions, from very low level of detail where only the overall shape of the system is visible, to high-resolution that approach, but not yet reach, atomic level of detail. In this dissertation, several modeling methods are introduced to either integrate cryo-EM datasets with structural data from X-ray crystallography, or to directly interpret the cryo-EM reconstruction. Such computational techniques were developed with the goal of creating an atomic model for the cryo-EM data. The low-resolution reconstructions lack the level of detail to permit a direct atomic interpretation, i.e. one cannot reliably locate the atoms or amino-acid residues within the structure obtained by cryo-EM. Thereby one needs to consider additional information, for example, structural data from other sources such as X-ray crystallography, in order to enable such a high-resolution interpretation. Modeling techniques are thus developed to integrate the structural data from the different biophysical sources, examples including the work described in the manuscript I and II of this dissertation. At intermediate and high-resolution, cryo-EM reconstructions depict consistent 3D folds such as tubular features which in general correspond to alpha-helices. Such features can be annotated and later on used to build the atomic model of the system, see manuscript III as alternative. Three manuscripts are presented as part of the PhD dissertation, each introducing a computational technique that facilitates the interpretation of cryo-EM reconstructions. The first manuscript is an application paper that describes a heuristics to generate the atomic model for the protein envelope of the Rift Valley fever virus. The second manuscript introduces the evolutionary tabu search strategies to enable the integration of multiple component atomic structures with the cryo-EM map of their assembly. Finally, the third manuscript develops further the latter technique and apply it to annotate consistent 3D patterns in intermediate-resolution cryo-EM reconstructions. The first manuscript, titled An assembly model for Rift Valley fever virus, was submitted for publication in the Journal of Molecular Biology. The cryo-EM structure of the Rift Valley fever virus was previously solved at 27Å-resolution by Dr. Freiberg and collaborators. Such reconstruction shows the overall shape of the virus envelope, yet the reduced level of detail prevents the direct atomic interpretation. High-resolution structures are not yet available for the entire virus nor for the two different component glycoproteins that form its envelope. However, homology models may be generated for these glycoproteins based on similar structures that are available at atomic resolutions. The manuscript presents the steps required to identify an atomic model of the entire virus envelope, based on the low-resolution cryo-EM map of the envelope and the homology models of the two glycoproteins. Starting with the results of the exhaustive search to place the two glycoproteins, the model is built iterative by running multiple multi-body refinements to hierarchically generate models for the different regions of the envelope. The generated atomic model is supported by prior knowledge regarding virus biology and contains valuable information about the molecular architecture of the system. It provides the basis for further investigations seeking to reveal different processes in which the virus is involved such as assembly or fusion. The second manuscript was recently published in the of Journal of Structural Biology (doi:10.1016/j.jsb.2009.12.028) under the title Evolutionary tabu search strategies for the simultaneous registration of multiple atomic structures in cryo-EM reconstructions. This manuscript introduces the evolutionary tabu search strategies applied to enable a multi-body registration. This technique is a hybrid approach that combines a genetic algorithm with a tabu search strategy to promote the proper exploration of the high-dimensional search space. Similar to the Rift Valley fever virus, it is common that the structure of a large multi-component assembly is available at low-resolution from cryo-EM, while high-resolution structures are solved for the different components but lack for the entire system. Evolutionary tabu search strategies enable the building of an atomic model for the entire system by considering simultaneously the different components. Such registration indirectly introduces spatial constrains as all components need to be placed within the assembly, enabling the proper docked in the low-resolution map of the entire assembly. Along with the method description, the manuscript covers the validation, presenting the benefit of the technique in both synthetic and experimental test cases. Such approach successfully docked multiple components up to resolutions of 40Å. The third manuscript is entitled Evolutionary Bidirectional Expansion for the Annotation of Alpha Helices in Electron Cryo-Microscopy Reconstructions and was submitted for publication in the Journal of Structural Biology. The modeling approach described in this manuscript applies the evolutionary tabu search strategies in combination with the bidirectional expansion to annotate secondary structure elements in intermediate resolution cryo-EM reconstructions. In particular, secondary structure elements such as alpha helices show consistent patterns in cryo-EM data, and are visible as rod-like patterns of high density. The evolutionary tabu search strategy is applied to identify the placement of the different alpha helices, while the bidirectional expansion characterizes their length and curvature. The manuscript presents the validation of the approach at resolutions ranging between 6 and 14Å, a level of detail where alpha helices are visible. Up to resolution of 12 Å, the method measures sensitivities between 70-100% as estimated in experimental test cases, i.e. 70-100% of the alpha-helices were correctly predicted in an automatic manner in the experimental data. The three manuscripts presented in this PhD dissertation cover different computation methods for the integration and interpretation of cryo-EM reconstructions. The methods were developed in the molecular modeling software Sculptor (http://sculptor.biomachina.org) and are available for the scientific community interested in the multi-resolution modeling of cryo-EM data. The work spans a wide range of resolution covering multi-body refinement and registration at low-resolution along with annotation of consistent patterns at high-resolution. Such methods are essential for the modeling of cryo-EM data, and may be applied in other fields where similar spatial problems are encountered, such as medical imaging.

Molecular cloning and characterization of an invertebrate cellular retinoic acid binding protein

We have cloned a cDNA and gene from the tobacco hornworm, Manduca sexta, which is related to the vertebrate cellular retinoic acid binding proteins (CRABPs). CRABPs are members of the superfamily of lipid binding proteins (LBPs) and are thought to mediate the effects of retinoic acid (RA) on morphogenesis, differentiation, and homeostasis. This discovery of a Manduca sexta CRABP (msCRABP) demonstrates the presence of a CRABP in invertebrates. Compared with bovine/murine CRABP I, the deduced amino acid sequence of msCRABP is 71% homologous overall and 88% homologous for the ligand binding pocket. The genomic organization of msCRABP is conserved with other CRABP family members and the larger LBP superfamily. Importantly, the promoter region contains a motif that resembles an RA response element characteristic of the promoter region of most CRABPs analyzed. Three-dimensional molecular modeling based on postulated structural homology with bovine/murine CRABP I shows msCRABP has a ligand binding pocket that can accommodate RA. The existence of an invertebrate CRABP has significant evolutionary implications, suggesting CRABPs appeared during the evolution of the LBP superfamily well before vertebrate/invertebrate divergence, instead of much later in evolution in selected vertebrates.

Molecular basis for the recognition of two structurally different major histocompatibility complex/peptide complexes by a single T-cell receptor

2C is a typical alloreactive cytotoxic T lymphocyte clone that recognizes two different ligands. These ligands are adducts of the allo-major histocompatibility complex (MHC) molecule H-2Ld and an endogenous octapeptide, and of the self-MHC molecule H-2Kb and another peptide. MHC-binding and T-cell assays with synthetic peptides in combination with molecular modeling studies were employed to analyze the structural basis for this crossreactivity. The molecular surfaces of the two complexes differ greatly in densities and distributions of positive and negative charges. However, modifications of the peptides that increase similarity decrease the capacities of the resulting MHC peptide complexes to induce T-cell responses. Moreover, the roles of the peptides in ligand recognition are different for self- and allo-MHC-restricted T-cell responses. The self-MHC-restricted T-cell responses were finely tuned to recognition of the peptide. The allo-MHC-restricted responses, on the other hand, largely ignore modifications of the peptide. The results strongly suggest that adaptation of the T-cell receptor to the different ligand structures, rather than molecular mimicry by the ligands, is the basis for the crossreactivity of 2C. This conclusion has important implications for T-cell immunology and for the understanding of immunological disorders.

A combined kinetic and modeling study of the catalytic center subsites of human angiogenin.

Kinetic analysis and molecular modeling have been used to map the ribonucleolytic center of angiogenin (Ang). Pyrimidine nucleotides were found to interact very weakly with Ang, consistent with the inaccessible B1 pyrimidine binding site revealed by x-ray crystallography. Ang also lacks an effective phosphate binding site on the 5' side of B1. Although the B2 site that preferentially binds purines on the 3' side of B1 is also weak, its associated phosphate subsites make substantial contributions: both 3',5'-ADP and 5'-ADP have Ki values 6-fold lower than for 5'-AMP, and adding a 3'-phosphate to the substrate CpA increases Kcat/Km by 9-fold. Thus Ang has a functional P2 site on the 3' side of B2 and a site for a second phosphate on the 5' side of B2. Modeling of an Ang-d(ApTpApA) complex suggested that Arg-5 forms part of the P2 site and that a 2'-phosphate might bind more tightly than a 3'-phosphate. Both predictions were confirmed kinetically. The subsite map obtained by this combined approach indicated that 5'-diphosphoadenosine 2'-phosphate might be a more potent inhibitor than any of the nucleotides tested thus far. Indeed, its Ki value of 150 microM is 50-fold lower than that for the best nucleotide previously reported and 400-fold lower than the Km for the best dinucleotide substrate. This compound may serve as a suitable starting point for the eventual design of tight-binding inhibitors of Ang as antiangiogenic agents for human therapy.

Similar antigenic surfaces, rather than sequence homology, dictate T-cell epitope molecular mimicry.

Molecular mimicry, normally defined by the level of primary-sequence similarities between self and foreign antigens, has been considered a key element in the pathogenesis of autoimmunity. Here we describe an example of molecular mimicry between two overlapping peptides within a single self-antigen, both of which are recognized by the same human self-reactive T-cell clone. Two intervening peptides did not stimulate the T-cell clone, even though they share nine amino acids with the stimulatory peptides. Molecular modeling of major histocompatibility complex class II-peptide complexes suggests that both of the recognized peptides generate similar antigenic surfaces, although these are composed of different sets of amino acids. The molecular modeling of a peptide shifted one residue from the stimulatory peptide, which was recognized in the context of the same HLA molecule by another T-cell clone, generated a completely different antigenic surface. Functional studies using truncated peptides confirmed that the anchor residues of the two "mimicking" epitopes in the HLA groove differ. Our results show, for two natural epitopes, how molecular mimicry can occur and suggest that studies of potential antigenic surfaces, rather than sequence similarity, are necessary for analyzing suspected peptide mimicry.

A molecular anchor for stabilizing triple-helical DNA.

Molecular modeling has been used to predict that 2,6-disubstituted amidoanthraquinones, and not the 1,4 series, should preferentially interact with and stabilize triple-stranded DNA structures over duplex DNA. This is due to marked differences in the nature of chromophore-base stacking and groove accessibility for the two series. A DNA foot-printing method that monitors the extent of protection from DNase I cleavage on triplex formation has been used to examine the effects of a number of synthetic isomer compounds in the 1,4 and 2,6 series. The experimental results are in accord with the predicted behavior and confirm that the 1,4 series bind preferentially to double- rather than triple-stranded DNA, whereas the isomeric 2,6 derivatives markedly favor binding to triplex DNA.

Antichagásicos potenciais: síntese e modelagem molecular de híbridos de hidrazonas e liberadores de óxido nítrico

A doença de Chagas é uma parasitose extremamente negligenciada, cujo agente etiológico é o protozoário Trypanosoma cruzi. Atualmente, 21 países da América Latina são considerados regiões endêmicas, onde 75-90 milhões de pessoas estão expostas à infecção, 6-7 milhões estão infectadas e mais de 41 mil novos casos surgem por ano. Entretanto, apenas os fármacos nifurtimox e benznidazol estão disponíveis no mercado. Estes, além da baixa eficácia na fase crônica da parasitose, apresentam diversos efeitos adversos, sendo que no Brasil apenas o benznidazol é utilizado. Este fato mostra a importância de se ampliar o número de fármacos disponíveis e propor quimioterapia mais eficaz para o tratamento da doença de Chagas. Como forma de contribuir para essa busca, este trabalho objetiva a síntese de compostos híbridos bioisostéricos N-acilidrazônicos e sulfonilidrazônicos, contendo grupo liberador de óxido nítrico, com potencial de interação com cisteíno-proteases parasitárias, tais como a cruzaína. Nestes derivados, os grupos liberadores de óxido nítrico utilizados foram os grupos furoxano (contendo substituinte metílico e fenílico) e éster nitrato. Propôs-se a variação de anéis aromáticos substituídos e não-substituídos, com o intuito de avaliar a possível relação estrutura-atividade (REA) desses análogos. Até o momento, somente os compostos da série N-acilidrazônica tiveram avaliação biológica realizada. Os valores de IC50 dos compostos na forma amastigota do parasita variaram entre >100 a 2,88 µM, sendo este último valor comparável ao fármaco de referência. A atividade inibitória frente à cruzaína foi de 25,2 µM a 2,2 µM. Já a liberação de óxido nítrico foi avaliada pelo método indireto de detecção de nitrato e os valores variaram entre 52,0 µM e 4.232,0 µM. Estes são bem inferiores ao composto padrão, além de não se identificar correlação direta entre a atividade biológica e a liberação de NO. Na sequência, os dois compostos mais ativos (6 e 14) foram submetidos a estudos de permeabilidade e de citotoxicidade. O composto 6 foi considerado o de maior permeabilidade segundo o Sistema de Classificação Biofarmacêutica (SCB) e todos os compostos apresentaram a taxa de fluxo menor que 2, indicando a ausência de mecanismo de efluxo. Na avaliação do potencial citotóxico desses compostos em células humanas, o derivado 6 apresentou índice de seletividade superior ao do benznidazol. Em estudos de modelagem molecular usando análise exploratória de dados (HCA e PCA), propriedades estéricas/geométricas e eletrônicas foram consideradas as mais relevantes para a atividade biológica. Além disso, estudos de docking mostraram que a posição do grupo nitro no anel aromático é importante para a interação com a cruzaína. Ademais o composto 6 não provocou mudanças significativas no ciclo celular e na fragmentação de DNA em células humanas, mostrando-se como líder promissor para futuros estudos in vivo. Atividade tripanomicida, citotoxicidade, potencial de liberação de NO e estudos de permeabilidade dos 23 derivados sulfonilidrazônicos e ésteres nitrato estão sendo avaliados.

Cytokine analysis in Atlantic bottlenose dolphins: Molecular characterization of IL-4, IL-6, and IL-10

The health status of wild and captive Atlantic Bottlenose dolphins ( Tersiops truncatis) is difficult to ascertain. Mass strandings of these animals have been attributed to pollutants, as well as bacterial infections. Using human Enzyme Linked Immuno-Assays (ELISA) for immunological cytokines, I measured soluble cytokine levels with respect to their health status. In a retrospective analysis of dolphin sera, there was a trend of higher cytokine levels in “sick” animals. I cultured dolphin lymphocytes in the presence of a mitogen (PHA), a super antigen (Staph-A), Lipopolysaccharide (LPS), and a calcium flux inducer (PMA). Levels of messenger RNA, from these cultured cells, were assayed with Polymerase Chain Reaction (PCR) using primers for the human cytokines IL-2, IL-4, IL-6, IL-10, Tumor Necrosis Factor, and Interferon gamma. Only IL-4, IL-6, and IL-10 messages were obtained, inferring similar nucleotide homology to the human primer sequences. The PCR products were sequenced. Sixteen IL-4 sequences, twelve IL-6 sequences and seven IL-10 sequences were obtained and analyzed. Each cytokine exhibited the same nucleotide sequence in all dolphins examined. There was no difference in the cytokine profile in response to the various stimuli. The derived amino acid composition for each of the dolphin cytokines was used for molecular modeling, which showed that dolphin IL-4, IL-6, and IL-10 were structurally similar to the corresponding proteins of Perissodactyla. ^

Mechanism‐based selection of a potent kallikrein‐related peptidase 7 inhibitor from a versatile library based on the sunflower trypsin inhibitor SFTI‐1

Potent and specific enzyme inhibition is a key goal in the development of therapeutic inhibitors targeting proteolytic activity. The backbone-cyclized peptide, Sunflower Trypsin Inhibitor (SFTI-1) affords a scaffold that can be engineered to achieve both these aims. SFTI-1's mechanism of inhibition is unusual in that it shows fast-on/slow-off kinetics driven by cleavage and religation of a scissile bond. This phenomenon was used to select a nanomolar inhibitor of kallikrein-related peptidase 7 (KLK7) from a versatile library of SFTI variants with diversity tailored to exploit distinctive surfaces present in the active site of serine proteases. Inhibitor selection was achieved through the use of size exclusion chromatography to separate protease/inhibitor complexes from unbound inhibitors followed by inhibitor identification according to molecular mass ascertained by mass spectrometry. This approach identified a single dominant inhibitor species with molecular weight of 1562.4 Da, which is consistent with the SFTI variant SFTI-WCTF. Once synthesized individually this inhibitor showed an IC50 of 173.9 ± 7.6 nM against chromogenic substrates and could block protein proteolysis. Molecular modeling analysis suggested that selection of SFTI-WCTF was driven by specific aromatic interactions and stabilized by an enhanced internal hydrogen bonding network. This approach provides a robust and rapid route to inhibitor selection and design.

Supramolecular organization in fluorene/indenofluorene-oligothiophene alternating conjugated copolymers

A series of conjugated copolymers containing fluorene or indenofluorene units alternating with oligothiophene segments, with potential interest for use as the active layer in field-effect transistors, is investigated. Atomic force microscopy analysis of the morphology of thin deposits shows either the formation of fibrillar structures, which are the signature of long-range π stacking, or the presence of untextured aggregates, resulting from disordered assembly. These morphologies are interpreted in terms of the supramolecular organization of the conjugated chains. Molecular modeling simulations indicate that the commensurability between the lengths of the monomer units and the presence of alkyl side groups are the two key structural factors governing the chain organization into highly ordered assemblies. The most favorable structures are those combining fluorene (indenofluorene) units with unsubstituted bithiophene (terthiophene) segments.

Jet and flash imprint defectivity - Assessment and reduction for semiconductor applications

Defectivity has been historically identified as a leading technical roadblock to the implementation of nanoimprint lithography for semiconductor high volume manufacturing. The lack of confidence in nanoimprint's ability to meet defect requirements originates in part from the industry's past experiences with 1 × lithography and the shortage in enduser generated defect data. SEMATECH has therefore initiated a defect assessment aimed at addressing these concerns. The goal is to determine whether nanoimprint, specifically Jet and Flash Imprint Lithography from Molecular Imprints, is capable of meeting semiconductor industry defect requirements. At this time, several cycles of learning have been completed in SEMATECH's defect assessment, with promising results. J-FIL process random defectivity of < 0.1 def/cm2 has been demonstrated using a 120nm half-pitch template, providing proof of concept that a low defect nanoimprint process is possible. Template defectivity has also improved significantly as shown by a pre-production grade template at 80nm pitch. Cycles of learning continue on feature sizes down to 22nm. © 2011 SPIE.

IgE cross-reactivity between Pas n 1 of Bahia grass pollen and other group 1 grass pollen allergens

Background The subtropical Bahia grass (Paspalum notatum) is an important source of pollen allergens with an extended season of pollination and wide distribution in warmer climates. The immunological relationship between pollen allergens of Bahia grass and temperate grasses is unresolved. Methods Serum IgE reactivity of grass pollen-allergic patients with Bahia, Ryegrass and Bermuda grass pollen extracts and their purified group 1 allergens, Pas n 1, Lol p 1 and Cyn d 1, were compared by immunoblotting, ELISA, inhibition ELISA, basophil activation by flow cytometry and molecular modeling. Results Differences in antibody recognition of allergenic components between Bahia grass and Ryegrass pollen were observed by immunoblotting. Eight grass pollen-allergic patients from a temperate region showed greater serum IgE reactivity with Ryegrass pollen than Bahia grass by ELISA. For seven of these sera, Ryegrass pollen inhibited IgE reactivity with Bahia grass pollen but not the converse. For 51 sera from grass pollen-allergic patients in this temperate region, IgE reactivity with Lol p 1 was greater than Pas n 1 or Cyn d 1. IgE reactivity with Lol p 1 was not inhibited by Pas n 1 or Cyn d 1, but Pas n 1 IgE reactivity was inhibited by Lol p 1. Two group 1 grass pollen allergen-specific mAb distinguished between temperate and subtropical grass pollens. Basophil activation for three patients tested was greater by Ryegrass pollen than Bahia or Bermuda grass, and by Lol p 1 than Pas n 1 or Cyn d 1. In contrast, two patients from a subtropical region had higher serum IgE reactivity with Bahia grass pollen than Ryegrass and Bahia grass pollen inhibited IgE reactivity with Ryegrass. A structural model of Pas n 1 showed amino acids implicated in IgE epitopes of other group 1 allergens were juxtaposed on the surface. Conclusion Allergens from subtropical Bahia grass pollen, including Pas n 1, share antigenic determinants with temperate grass pollen allergens, but patients exhibit higher serum IgE reactivity to their locally predominant grass pollen. Basophil activation by Bahia grass pollen and Pas n 1 in patients from a temperate climate indicates clinically relevant cross-sensitization between temperate and subtropical grass pollens.

Void Geometry Driven Spin Crossover in Zeolite-Encapsulated Cobalt Tris(bipyridyl) Complex Ion

The cobalt(II) tris(bipyridyl) complex ion encapsulated in zeolite-Y supercages exhibits a thermally driven interconversion between a low-spin and a high-spin state-a phenomenon not observed for this ion either in solid state or in solution. From a comparative study of the magnetism and optical spectroscopy of the encapsulated and unencapsulated complex ion, supported by molecular modeling, such spin behavior is shown to be intramolecular in origin. In the unencapsulated or free state, the [Co(bipy)(3)](2+) ion exhibits a marked trigonal prismatic distortion, but on encapsulation, the topology of the supercage forces it to adopt a near-octahedral geometry. An analysis using the angular overlap ligand field model with spectroscopically derived parameters shows that the geometry does indeed give rise to a low-spin ground state, and suggests a possible scenario for the spin state interconversion.